关于OpenGL的前后台问题:
Background Apps May Not Execute Commands on the Graphics Hardware
An OpenGL ES app is terminated if it attempts to execute OpenGL ES commands on the graphics hardware. iOS prevents background apps from accessing the graphics processor so that the frontmost app is always able to present a great experience to the user. Your app can be terminated not only if it makes OpenGL ES calls while in the background but also if previously submitted commands are flushed to the GPU while in the background. Your app must ensure that all previously submitted commands have finished executing before moving into the background.
If you use a GLKit view and view controller, and only submit OpenGL ES commands during your drawing method, your app automatically behaves correctly when it moves to the background. The GLKViewController class, by default, pauses its animation timer when your app becomes inactive, ensuring that your drawing method is not called.
If you do not use GLKit views or view controllers or if you submit OpenGL ES commands outside a GLKView drawing method, you must take the following steps to ensure that your app is not terminated in the background:
In your app delegate’s applicationWillResignActive: method, your app should stop its animation timer (if any), place itself into a known good state, and then call the glFinish function.
In your app delegate’s applicationDidEnterBackground: method, your app may want to delete some of its OpenGL ES objects to make memory and resources available to the foreground app. Call the glFinish function to ensure that the resources are removed immediately.
After your app exits its applicationDidEnterBackground: method, it must not make any new OpenGL ES calls. If it makes an OpenGL ES call, it is terminated by iOS.
In your app’s applicationWillEnterForeground: method, re-create any objects and restart your animation timer.
To summarize, your app needs to call the glFinish function to ensure that all previously submitted commands are drained from the command buffer and are executed by OpenGL ES. After it moves into the background, you must avoid all use of OpenGL ES until it moves back into the foreground.